1. Field of the Invention
This invention pertains to a retaining structure of caissons for use in building islands in bodies of water. More specifically, the invention pertains to a stressed caisson retaining system for building an island structure for use as an oil or gas well drilling and production platform in Arctic waters subject to impingement by ice formations.
2. Description of the Prior Art
In the shallow water Arctic regions off the northern coasts of Alaska and Canada, man-made islands have been proposed for use in supporting oil and gas exploration and production facilities. In fact, a number of islands have been built in the Beaufort Sea and exploration wells have been drilled from those islands. Most of those islands were built from fill material placed on the bottom of the body of water. The islands generally have gradually sloping beaches for erosion protection. Large amounts of fill are needed to build islands in this manner. The amounts of fill material and the costs of the islands increases dramatically with small increases in the water depth. Additionally, in many areas of the Arctic, fill material is not readily accessible and must be transported great distances to the island construction site again causing the cost of the island to increase dramatically with the amount of fill material needed.
It has been proposed to use conventional sheet pile enclosures or even a prefabricated cylindrical retaining wall to give the island a more cylindrical shape and thereby reduce the amount of fill material needed. Islands retained by sheet pile enclosures have definite limitations in Arctic applications. The greatest problem with a sheet pile enclosed island is in its foundation. The sheet pile foundation is extremely susceptible to slide failures as a result of the high differential loading of the sea bed inside the sheet pile enclosure compared to outside the enclosure. This means that the lower ends of the sheet piles would be subjected to great forces tending to move those lower ends outward from the island. Additionally, a great deal of time and equipment is needed to drive the sheet piles. In the Arctic, the open water period for offshore construction can be extremely short and seldom exceeds 80 days.
The prefabricated cylindrical retaining wall concept utilizes a large unitary tank member. It would normally require a diameter on the order of 300 feet. Transportation of the tank unit to the Arctic region would present a formidable problem. Even moving the tank from one exploration drilling site in the Arctic to the next drilling site could be difficult. Perhaps its biggest drawback, however, is that the continuous horizontal loads imposed upon the island by ice formations could in time deform the tank member since the fill material retained within the tank will deform slightly with time, leaving only the tank itself to resist the horizontal load. This lack of flexibility in the retaining structure would conceivably result in frequent damage to the tank.
Therefore, there exists a need for a portable retaining structure which can rapidly be assembled and is capable of withstanding the great forces imposed upon it by ice sheets during winter and by wave action during the open water period.